A non-impact printing process can be simply defined as a process which uses an electronic, electric, magnetic or optical means to produce characters as opposed to a mechanical means. Of the non-impact printing processes, there is a group of printing methods that uses electrostatic techniques. Electrostatic printing can be defined as those methods which use the interaction of electrostatically charged marking particles and an electric field to control the deposition of the marking particles onto a substrate, and encompasses processes generally known as electrographic, electrophotographic, or electrostatographic printing.
Electrostatography can be a term used to describe the various non-impact printing processes which involve the creation of a visible image by the attraction of charged imaging particles or marking particles to charged sites present on a substrate. Such charged sites, forming what is usually termed a latent image, can be transiently supported on photoconductors or pure dielectrics and may be rendered visible in situ or be transferred to another substrate to be developed in that location. Additionally such charged sites may be the reflection of those structured charges existing within a permanently polarised material as in the case with ferroelectrics or other electrets.
In electrostatography the imaging particles, generally known as toner, can be of the dry type or of the liquid type. Dry powder toners have many disadvantages. For example the performance of dry powder toners is very susceptible to environmental conditions, influencing, for example, charge stability, and therefore giving rise to variable image performance. Also, the large particle size of dry powder toners is a major contributing factor in not allowing the achievement of highly resolved developed images.
For high speed, long run printing, cost per page is a principal consideration. In particular, the cost of fusing the image to paper or any other desired substrate significantly contributes to the running costs of such a printer. Other objections are related to the problem of dusting. Dust or fine or small particles of toner are prone to escape from the developer, and these deposit onto any surface both within and outside the printing device, causing mechanical failures within the device and environmental problems outside the device. This problem becomes severe when such dry powder printing devices are run at high speed. In addition, achieving high resolution with dry powder toners at high speed is difficult due to the fact that the dusting problem is further exacerbated by the need to reduce dry toner particle size to a level which will allow acceptable resolution at high speeds, which further compounds the difficulty and dangers in handling such fine powders. Dry powder system therefore can not in practice achieve high resolution images at high speeds, that are usually associated with analogue printing methods such as off-set and gravure printing. Other disadvantages include cost of the general maintenance of the printer and cost of the dry powder toner.
It is known that latent electrostatic images can be developed with marking particles dispersed in insulating or non-polar liquids. Such marking particles normally comprise colouring matter such as pigments which have been ground with or otherwise combined with resins or varnishes or the like. Additionally, charge directing agents are usually included to control the polarity and charge-to-mass ratio of the toner particles. These dispersed materials are known as liquid toners or liquid developers. In use, a liquid developer is applied to the surface of a latent image bearing member to develop an electrostatic image on the member.
Liquid toner development systems are generally capable of very high image resolution because the toner particles can safely be much smaller, normally in the range of 0.5 to 3 μm, than dry toner particles which are normally in the range of 7 to 10 μm. Liquid toner development systems show impressive grey scale image density response to variations in image charge and achieve high levels of overall image density. Additionally, the systems are usually inexpensive to manufacture and are very reliable. Furthermore, the liquid toners for these systems are operationally and chemically stable, particularly to environmental changes due to buffering properties of the carrier liquid, thus exhibiting a particularly long shelf-life.
Liquid developers have generally utilized low viscosity liquids and low concentration of the solids, that is, of marking particles. These traditional toners and associated process systems may be termed low viscosity toner or LVT systems. Generally, LVT systems utilise toners with low viscosities, typically 1 to 3 mPa·s. and low volumes of solids, typically 0.5 to 2% by weight. Maintaining a uniform dispersion of the marking particles can be difficult in a low viscosity toner system. The marking particles have a tendency to drift and settle in the carrier liquid. Furthermore, low volume of solids in the toner increases the amount of toner required to develop a given latent image. More liquid toner will have to be presented to the photoconductor surface in order to provide sufficient marking particles for a desired image density. In order to meet this toner supply demand, LVT printing systems are usually designed to have reasonably large development gaps. Such an arrangement of the development region has several drawbacks, such as a reduced strength and uniformity of the electric field in the development gap, and additional complexity in the design required to maintain a constant gap in the printing direction, as well as across the page. This usually results in reduced development efficiency, edge effects and non-uniform solid fill.
Devices using such liquid electrographic printing can also have some objectionable problems, especially when these devices are required to operate at speeds at or above 0.5 ms−1. The main problem is in regard to the solvent carry-out. The term solvent carry-out relates to the quantity of solvent or carrier which is transferred onto and trapped within the paper. Such solvent subsequently evaporates during image fusing, giving rise to atmospheric pollution and also adding significantly to production costs. A further disadvantage of such liquid toning is the tendency for deposition of colouring matter in non-image or background areas which results in a general discolouration of the copy, normally referred to as background staining or fog.
To overcome these and other known problems that can be associated with LVT systems, highly concentrated liquid toner development systems utilising toner with solids concentrations of up to 60% by weight and viscosities of up to 10,000 mPa·s, and utilizing thin films, typically 1 to 40 μm, of the highly concentrated and viscous liquid toner have been disclosed. This system of developing electrostatic latent images with these viscous and highly concentrated liquid toner systems may be termed high viscosity toner or HVT systems. Examples of such liquid toners are disclosed in commonly assigned U.S. Pat. No. 5,612,162 to Lawson et al., and U.S. Pat. No. 6,287,741 to Marko, the disclosures of which are totally incorporated herein by reference. Examples of high viscosity, high concentration liquid developing methods and apparatus are disclosed in commonly assigned U.S. Pat. No. 6,137,976 to Itaya et al. and U.S. Pat. No. 6,167,225 to Sasaki et al., the disclosures of which are totally incorporated herein by reference. These new HVT liquid developing systems overcome many of the short-comings of traditional LVT systems. The term high viscosity is intended to refer to viscosities of the prepared toner of greater than 10 mPa·s., and a solids concentrations of up to 60% by weight.
In the liquid development of electrostatic latent images by LVT systems, the electrostatic latent images formed on the image bearing member are made into visible images by the toner, which consists of charged marking particles in an insulative liquid. Some such LVT systems may use the same carrier medium, as used in the liquid developing agent, to apply a pre-wet liquid on the image bearing member before the actual developing process begins; this is a well known means of preventing the adhesion of toner to the non-image parts of the image bearing member and thereby preventing background staining or fog. In most instances, however, the use of a pre-wet liquid in LVT systems is not required due to the fact that liquid toners used in such systems are of a low solids concentration and of low viscosity.
Traditionally, HVT printing systems have utilised pre-wet mechanisms to minimise background staining or fog, due to the fact that HVT type systems utilise liquid toners of very high solids content and of high viscosity. Various methods have been disclosed which can be used to apply the pre-wet liquid. For example, a roller with depressions and protuberances may be used as the member that supplies the pre-wet liquid. Alternatively, a blade provided with a slit from which pre-wet liquid flows may be used. In this method of applying the pre-wet liquid, the blade is positioned near to the image bearing member such that the pre-wet liquid forms a liquid bank between the image bearing member and the blade. In most instances however, the mechanical application of a pre-wet liquid can be problematic in that it requires high precision in dispensing a small and controlled amount of liquid in order to achieve background fog prevention over the whole printing area. It may therefore be difficult to adequately prevent toner adhesion to the non-image parts on the image bearing member. This problem is further exacerbated at high speeds. Further, the pre-wet liquid may have different physical and or chemical properties to those of the carrier fluid of the liquid toner. In those cases, there can be associated difficulties in recycling the liquid developer contaminated with the pre-wet liquid.
At high speeds, processing parameters and development times become much more critical and special constructions and operational techniques are necessary for good imaging. The HVT systems have been further developed and it is an object of this invention to provide a method and means for high speed electrographic printing utilising highly viscous, highly concentrated liquid developers. Additionally, there is a strong desire for a high speed, highly concentrated liquid toner development system that can operate at high speed whilst achieving high print image density, no background staining or fog, and without the need for the separate mechanical application of a pre-wet to the imaging member prior to latent image development.
It is a further object of this invention to provide a method and means for high speed electrographic printing utilising highly viscous, high solids content liquid developers that achieve highly resolved images at high speeds, that are usually associated with analogue printing methods such as offset and gravure printing.
The term “high speed” as herein used is intended to mean printing speeds of greater than 0.5 ms−1.